Stainless steel

ABSTRACT

1. A QUENCH HARDENABLE STRAIGHT-CHROMIUM, LOW CARBON STAINLESS STEEL (COMPRISING) ESSENTIALLY CONSISTING OF 10.0% TO 14.0% CHROMIUM, .07% TO .14% CARBON, (AND REMAINDER SUBSTANTIALLY ALL IRON) IN WHICH THE RESPONSE IN HARDNESS OF THE METAL TO VARIATION IN TEMPERING TEMPERATURE THEREOF IS APPRECIABLY RETARDED AND IMPROVED TOUGHNESS IN THE HARDENED AND TEMPERED CONDITION IS HAD, THROUGH THE INCLUSION IN THE METAL OF .05% TO .35% COLUMBIUMTANTALUM, AND 0% TO .20% VANADIUM, THE PRECISE AMOUNT OF (THESE INGREDIENTS WITHIN THESE RANGES) COLUMBIUMTANTALUM BEING ABOUT ONE TO (THREE) TWO TIMES THE CARBON CONTENT, AND REMAINDER SUBSTANTIALLY ALL IRON.

United States Patent 27,671 STAINLESS STEEL Harry Tanczyn, Baltimore,Md., assignor to Armco Steel Corporation No Drawing. Original No.3,000,729, dated Sept. 19, 1961, Ser. No. 856,925, Dec. 3, 1959.Application for reissue Mar. 1, 1971, Ser. No. 111,774

Int. Cl. C22c 39/14 US. Cl. 75--126 F 5 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

My invention, generally relating to stainless steels, has particularapplication to the straight chromium grades, especially the well-knownA.I.S.I. Type 410, the composition of which is given below.

An object of my invention is to provide a hardenable stainless steel ofthe character indicated which is readily amenable, in simple and directmanner, to the realization of desired intermediate hardness values inclose response to commercial specifications.

A further object is to provide a stainless steel of the general typedescribed wherein, without substantial sacrifice of hardness andstrength, there are achieved improved toughness and improved impactstrength, all at minimum cost and in simple, direct and reliable manner.

Other objects and advantages in part will be obvious and in part morefully pointed out during the course of the following disclosure.

My invention, accordingly, resides in the combination of elements,composition of ingredients, and in the re lation of each of the same toone or more of the others, all as described herein, the scope of theapplication of which is indicated in the claims at the end of thisspecification.

As conducive to a more ready understanding of my invention, it may benoted at this point in the disclosure that the straight chromium gradesof heat-hardenable stainless steels basically are of martensiticstructure. They are comparatively inexpensive and of widespread utility.Particularly is this true of the A.I.S.I. Type 410, this analyzing 0.15%max. carbon, 1.00% max. manganese, 1.00% max. silicon, 0.040% max.phosphorus, 0.030% max. sulphur, 11.50% to 13.50% chromium, andremainder iron.

The steel of above analysis is well suited for general use whereresistance to corrosion and/or to heat is a requisite. As well, thissteel as presently produced, employs a minimum of critical and costlyalloying ingredients. It displays only fair machining properties,however.

Characteristic in the use of steels of the general type noted is aschedule of thermal treatments, this including rapid cooling from ahardening temperature of say, about 1800" F. and a subsequent temperingtreatment. As hardened from 1800 F. the steel displays a hardness in theapproximate range of 43 to 44 Re. It is to be noted, however, that bysuitable variations in the tempering treatment a rather wide range ofmechanical properties may be developed.

Customarily, and following hardening resort is made to a temperinganneal at somewhat lower temperature. This serves both to relievestresses and, incident to hardening, perhaps to increase the toughnessof the steel, while substantially retaining required hardness. However,it is this tempering process which is relied upon to control the precisehardness of the particular steel for its intended purpose, and to bringthe same to a selected value somewhat below its maximum value. As notedabove, the maximum hardness ordinarily is in the range of Re 43-44.

Now, the very fact that the Type 410 is of general utility hasfrequently caused the customer to specify within close limits both thecomposition and the mechanical "ice properties of the steel which heorders. A frequent requirement is that the steel be processed at themill to an intermediate hardness value within the narrow range, ofapproximately Rc 26-32.

In practice, however, it is extremely difficult to bring the steelwithin such narrow limits. This is largely because of the rapid changeof hardness which takes place when the steel. is subjected to atempering operation; the ameliorating ettect is unduly dependent uponrelatively small difierences in the temperature at which the temperingoperation is conducted. The steel is much too sensitive. Thus in thepresent condition of the art, and in order to closely approximate therequired hardness it frequently is necessary to reheat the metal severaltimes during the tempering process, and this by trial and error, firstat one temperature and then at another, until the specified range ofhardness is had. This practice is tedious and time-consuming, withattendin expense represented in tie-up of labor and equipment. Thesedifiiculties are particularly severe when, as is usually the case, alarge tonnage of steel in the form of sheet, strip, plate, bars, rods orwire is undergoing treatment. For here, uniform temperature both withinthe batch and from one batch to another, is rendered ditficult, it notimpossible. And so is the response to tempering treatment.

Another characteristic disadvantage observed in the straight chromiumsteels is a larger grain structure and a comparatively low value oftoughness. That is, these steels are found to be normally somewhatbrittle. Thus, and typically, the steels are found to have a maximum ofonly about 40 ft. lbs. Charpy. It, of course, is apparent that anysubstantial increase in toughness which could be brought about wouldappreciably increase the field of application of the steels. And, inturn, this would importantly increase or enhance economic acceptance.

An important object of my invention, therefore, is to provide in directand simple manner, with comparatively limited additional cost, a steelwherein hardness may be positively brought to selected intermediatevalues, and this in but a single tempering treatment, while the grainsize of the steel is materially lowered and both toughness andresistance to impact are importantly increased and controlled.

Referring now to the practice of my invention, I have found that theaddition to the straight chromium grades of stainless steel, ofcomparatively small yet closelycontrolled quantities of columbium willappreciably reduce the criticality of the change in hardness withvariations of temperature in a stress-relieving tempering treatment; abroader range of tempering temperatures results, this assuringsubstantially the same hardness values in spite of differences in thetempering temperatures. These superior results are most surprising, andare vividly accented, when it is considered that although the prior artdiscloses many instances where columbium has been added to stainlesssteel, in no instance have my advantageous results attended suchpractice. The amount of the columbium addition in my steel is highlycritical, especially as it relates to the carbon content, all as appearsmore fully hereinafter. Columbium is costly material but since thequantity of columbium required is so small, the relatively high cost ofthis material is not prohibitive.

As a practical matter, tantalum usually is employed along with thecolumbium. I find that the addition of a small amount of tantalum insubstitution for a corre sponding part of the columbium additive,results in no appreciable diminution of advantageous results had bycolumbium alone (that is, without an admixture of tantalum) as aconstituent of the steel. At the same time, and since tantalum is muchless expensive than columbium, this significantly reduces the cost ofthe alloying ingredients.

Columbium introduced into the steel within the critical narrow limitsnot only gives superior control over the precise intermediate hardnessvalue realized through tempering and assures uniform hardness even withan appreciable spread in the temperatures actually attained in a batchof steel being treated, but also gives an important increase in thetoughness of the steel which is uniform throughout the batch.Furthermore, microscopic study discloses an importantly reduced size ofgrain within the metal; a more ductile product results.

My further investigations have disclosed that vanadium, as a replacementfor part of the columbium, will likewise improve the characteristics oftemper hardness, as against the precise temperature of the temperingtreatments. However, vanadium itself produces no beneficial effect oneither the grain-size of the steel or the toughness; in fact vanadiuminduces brittleness. Perhaps best results at minimum cost are had withcolumbium and vanadium together (or columbium along with tantalum whencombined with vanadium). With the combination of ingredients it is thecolumbium, or mixture thereof with tantalum, which controls the size ofthe grain, while it is both the vanadium and columbium additive (orcolumbium together with tantalum) which broadens the hardness control.In combining the ingredients vanadium is present in about the sameamount as the columbium alone or as the columbium along with tantalum.

Typically, in the practice of my invention, columbium and tantalum maybe present in amount ranging from about 0.05% to about 0.35% of thesteel, with vanadium present in amount ranging from about 0.05% to about0.20%.

Broadly, the steel of my invention essentially consists of about [70%]to .14% carbon, .10% to 1.25% manganese, 001% to 050% phosphorus, 001%to .050% sulphur, 10% to 1.00% silicon, 10.0% to 14.0% chromium, 1.00%max. nickel, .05% to .35% columbium and tantalum together, without orwith vanadium in the amount of 05% to .20%, the columbium, tantalum andvanadium together amounting to .05% to 35%, and remainder substantiallyall iron. Molybdenum, tungsten and nitrogen are present as residuals,the molybdenum and tungsten each in amounts not exceeding about 20%,however. and nitrogen in amounts up to about 05%.

To further illustrate the practice of my invention, it may be noted thata preferred steel, employing as an additive columbium along withtantalum, analyzes approximately as follows: Carbon .12%, manganese,50%, phosphorus 010%, silicon 35%, sulphur 010%, chromium 12.0%, nickel20%, columbium and tantalum together 15%, and remainder iron. Anothersuch preferred steel analyzes about the same, except that the amount ofcolumbium together with tantalum is lowered to about 0.12%, whilevanadium is included in amount of about 0.10%. The precise analysis isas follows: Carbon .l2%, manganese 50%, phosphorus 010%, sulphur 010%,chromium 12.0%, silicon 35%, nickel 20%, columbium and tantalum .l2%,vanadium .10%, and remainder iron.

My stainless steel is hardened upon cooling from a temperature range ofsay about 1650" to 1950 F., enduring for about four hours. Cooling isbad either in air or by quenching in oil or water. Following suchhardening, the metal is then subjected to further heat treatment assuggested above, both for the relief of stresses and to impart requisitetemper to the metal. This is had by reheating the steel at a temperaturebetween about 500 and 1200 F.

As specifically illustrative of the practice of my invention I madethree heats of stainless steel with chemical composition as set forth inTable I below, two according to my invention (heats 2305 and 2306) andone, for comparison purposes, according to the prior art (heat 2305):

1 Residual.

Samples of the three steels of Table I were then hardened and tempered,this latter under three different conditions conveniently identified asA, B and C. A schedule of the three hardening and tempering treatmentsis as follows:

Al800 F. /2 hr.--oil quench and 1125 F. 4 hours air cool.

B1800 F. /2 hr.oil quench and 1200 F. 4 hrs.

air cool.

C1800 F. /2 hr.oil quench and 1200 F. 4 hrs.air

cool and 1100" F. 4 hrs-air cool.

More particularly, in A the several samples of the three steels werehardened by heat-treatment at 1800 F., enduring for about half an hour,followed by an oil quench. I thereupon tempered the samples at atemperature of about 1125 F. for a period of approximately four hours.Following this, I cooled the metal in air. In B the tempering wasaccomplished at about 1200 F. In all other respects, however, treatmentB corresponds with treatment A. And treatment C followed the procedureof treatment B, but in addition, the samples were given a subsequent andsecond tempering treatment, this at a reduced temperature of about 1100F. This second tempering operation endured for a period of about 4 hoursfollowing which the samples were cooled in air.

The mechanical properties of the three steels of Table 1, these underthe three conditions of heat-treatment A, B and C are given in Table IITABLE I I Mechanical properties of the three steels of Table I Charpy0.2% Percent Percent; V -noteh U.'I.S Y.S. el. in red. impact Heat No.Cond. p.s.l p.s.i. 2" area ft./lbs.

The test results given above clearly establish that under all threeconditions of heat-treatment the strength of the steel is greater wherecolumbium-tantalum has been added, in controlled amount, according tothe practice of my invention. Moreover, and surprisingly so, there is agreat increase in the impact strength of my steel as compared to thesteel of the prior art. Actually, the notch impact strength is found tohave been more than doubled.

Moreover, upon comparing the results in condition B with those ofcondition A, it will be observed that while there is some reduction intensile strength, the toughness of the metal is materially improved. Ingeneral, the small sacrifice in ultimate tensile strength and yieldstrength is found to be unimportant, as compared to the great increasein toughness as indicated by the V-notch impact figures.

Upon comparing the mechanical properties of my steel in conditions C andB, it is seen that little or nothing is gained by the further temperingtreatment condition C. Actually, the first tempering is accompanied byslight decrease in strength values, together with some slight loss ofimpact strength. Thus, through the inclusion of columbium-tantalum onlya single tempering treatment is required to bring about nice control ofthe strength and hardness, together With material improvement in impactstrength.

And as a further matter, while the columbium-tantalum-containing steelsof my invention were observed to have a fine grain structure, acomparatively coarse grain structure characterized the commercial Type410 of the pier art. This coarse grain structure was vitually unchanged,upon inclusion of a generally like amount of vanadium. And this sameresult was observed where vanadium was added along with an admixture oftungsten.

And finally, I obtain in my steel a consistency in the temper ofhardness had, throughout a batch of sheet, strip, wire, or the like, inspite of temperature differences which are reached throughout the batch.For in a batch of commercial 410 stainless steel wire the temperedhardness was observed to range in value from about Rockwell 13C to 20C.Such values are bad, not only because they are low, but because of theirwide range. In my steel, however, in which columbium is present with orwithout vanadium, the hardness variation from batch to batch was foundto range in value only about Rockwell 2C. Such close adherence tospecification is well within acceptable commercial limits.

The amount of columbium employed in my steel (or columbium plustantalum) is in every sense critical. For where the columbium content islower than .05% no benefit is had. And where the additive is present inpercentages beyond those heretofore disclosed, that is, in excess of.35%, disappointing results are observed. Sharp loss in strengthqualities are noted. For satisfactory results, it is best that thecolumbium (or columbium and tantalum) does not exceed about three timesthe carbon content of the steel. Even better results are obtained whenthis ratio is limited to not more than about one or two times the carboncontent.

The vanadium content, where such addition is made, likewise is critical;with vanadium exceeding .15% and a total of columbium-tantalum andvanadium exceeding about 35%, a loss of strength is noted. And of evengreater consequence, with an excess of these ingredients, andparticularly where there is residual molybdenum exceeding .20% and thesum of molybdenum, columbiumtantalum and vanadium exceeds .45%, theforgeability suffers (there is an inclination to split in forging). Thetransverse ductility likewise suffers. This I attribute to an excess ofdelta-ferrite.

I prefer to employ vanadium along with the columbium-tantalum because Ifind that, for some reason unknown to me, a lesser quantity of the groupis required than with the columbium-tantalum alone.

As to the reason why the desirable results of fine grain structure, highimpact strength and consistent response to tempering treatment are hadin the steel of my invention, reflection suggests that the columbiumpresent combines with much of the carbon present to form columbiumcarbides. These carbides are not readily soluble in the steel at thetemperatures of heat-treatment or even the temperatures of hot-Working.Rather they are distributed throughout the metal, and in the same mannerinhibit grain growth, perhaps serving as nuclei for new grains with theresult that there are more grains but these of smaller size. As notedabove, the desired results are had with or without vanadium. Theseresults, however, are not had with vanadium alone. The vanadium carbideswhich form are considerably more soluble in the steel and presentnothing to inhibit grain growth. I am by no means certain, however, ofthe precise cause of the results which are obtained. By consequence, Ido not desire to be bound by the theoretical explanation here offered.Suffice it to say that my desirable results are faithfully achieved, andthis, with the certainty requisite for commercial feasibility.

The loss of strength and impact values which accompany an excess ofcolumbium and tantalum is attributed to a tendency of these ingredientsto tie up the carbon within the metal, and to take the same out ofsolution. This sacrifices the hardenability of the metal, as well as itsresultant hardness and strength.

Thus it will be seen that I provide in my invention a straight chromiumgrade of hardenable stainless steel in which there is had the variousobjects hereinbefore set forth together with many practical advantages.The steel of my invention reliability responds to temper treatment togive improved ductility, fine grain structure and high impact strength.Moreover, it favorably responds to tempering treatment, which responsechanges less sharply with temperature than the steel of the prior art.

All the foregoing, as well as many other practical advantages attend thepractice of my invention.

It is apparent that, once disclosed many modifications of the presentembodiments of my invention will suggest themselves to those skilled inthe art. And, similarly, that many embodiments of the underlyinginventive thought will likewise come to mind, all falling within thescope of this disclosure. Accordingly, I intended the foregoingdescription to be considered as simply illustrative, and not ascomprising limitation.

I claim as my invention:

1. A quench hardenable straight-chromium, low carbon stainless steel[comprising] essentially consisting of 10.0% to 14.0% chromium, 07% to.14% carbon, [and remainder substantially all iron] in which theresponse in hardness of the metal to variation in tempering temperaturethereof is appreciably retarded and improved toughness in the hardenedand tempered condition is had, through the inclusion in the metal of.05% to .35% columbiumtantalum, and 0% to .20% vanadium, the preciseamount of [these ingredients within these ranges] columbiumtantalumbeing about one to [three] two times the carbon content, and remaindersubstantially all iron.

2. A quench hardenable straight-chromium stainless steel of improvedtoughness and strength in the hardened and tempered condition, saidsteel essentially consisting of carbon 07% to .14%. manganese .10% to1.25%, phosphorus 001% to 050%, sulphur 001% to 050%, silicon .10% to1.00%, chromium 10.0% to 14.0%. nickel 1.00% maximum, columbium andtantalum .05% to 25%, vanadium .05% to .20%, the sum total of columbium,tantalum and vanadium additions amounting to about .05% to 35%, and theremainder substantially all iron.

3. A quench-hardenable straight-chromium stainless steel of improvedtoughness and strength in the hardened and tempered condition, saidsteel essentially consisting of carbon 07% to .14%, chromium 10.0% to14.0%, columbium and tantalum .05% to .35%. vanadium .05% to .20%, withthe sum of columbium and vanadium amounting to .05% to 35% and about oneto three times the carbon content. and the remainder substantially alliron.

4. A quenchhardenable straight-chromium stainless steel of improvedtoughness and strength in the hardened and tempered condition, saidsteel essentially consisting of carbon about .12%, chromium about 12.0%,columbium and tantalum about .12% to .15%, vanadium 0% to about 10%, andthe remainder substantially all iron.

5. A straight-chromium stainless steel having quenchhardening propertiesand wherein temper hardness can be closely controlled within requiredvalue wherein impoved toughness and strength are bad in the hardened andtempered condition, said steel comprising about 10.0% to 14.0% chromium,about 0.07% to 0.14% carbon, about 0.10% to 1.25% manganese, about0.001% to 0.050% phosphorus, about 0.001% to 0.050% sulphur, about 0.10%to 1.00% silicon, nickel up to 1.00%, co-

7 lumbium and tantalum together in the amount of 0.05% UNITED STATESPATENTS to 0.35%, vanadium about 0.05% to 0.15%, with colurnbium andtantalum together with vanadium in sum total 2469887 5/1949 Olcott75-426 about .05 to 35%, and up to about three times the car R N PATENTSbon content of the steel, and the remainder substantially 5 741 93 2 9 5Great Britain all iron.

References Cited HYLAND BIZOT, Primary Examiner The followingreferences, cited by the Examiner, are of record in the patented file ofthis patent or the original patent.

US. 01. X.R. 10 75126 E

1. A QUENCH HARDENABLE STRAIGHT-CHROMIUM, LOW CARBON STAINLESS STEEL(COMPRISING) ESSENTIALLY CONSISTING OF 10.0% TO 14.0% CHROMIUM, .07% TO.14% CARBON, (AND REMAINDER SUBSTANTIALLY ALL IRON) IN WHICH THERESPONSE IN HARDNESS OF THE METAL TO VARIATION IN TEMPERING TEMPERATURETHEREOF IS APPRECIABLY RETARDED AND IMPROVED TOUGHNESS IN THE HARDENEDAND TEMPERED CONDITION IS HAD, THROUGH THE INCLUSION IN THE METAL OF.05% TO .35% COLUMBIUMTANTALUM, AND 0% TO .20% VANADIUM, THE PRECISEAMOUNT OF (THESE INGREDIENTS WITHIN THESE RANGES) COLUMBIUMTANTALUMBEING ABOUT ONE TO (THREE) TWO TIMES THE CARBON CONTENT, AND REMAINDERSUBSTANTIALLY ALL IRON.